1. Field of the Invention
The present invention is related to flat panel information displays and, more specifically, to FED (Field Emission Display) devices based on electron emission from sharp conducting objects.
2. Description of the Related Art
It is already known that strong electric fields, in the order of megavolts per centimeter, can be used to provide cold cathode emission from conducting surfaces. It is also well known that when the emitters are shaped as sharp needles or edges, the emission voltage can be reduced to more practical levels, such as a few kilovolts per centimeter.
Such effect can be efficiently used for fashioning electronic devices which operate like electronic valves or, better, as cathode ray tubes, with the advantage of eliminating the cathode heating and saving the power needed when compared to the latter, thus increasing the overall efficiency. Such a device is described in U.S. Pat. No. 3,789,471 which shows structures that function as diodes and triodes, where the cathode is shaped like a sharp tip in which the concentration of the electric field produces cold cathode electron emission. The manufacturing process for such electrodes was first described by Spindt in 1968, hence those electrodes are known as Spindt emitters.1 
1Spindt, C. “A Thin-Film Field Emission Cathode”, Journal of Applied Physics, Vol. 39, No. 7, June 1968 
As shown in the above mentioned patent document, as well as in U.S. Pat. No. 3,665,241, the electron source comprises a plurality of Spindt emitters, conical or pyramidal, placed over a conducting substrate, with the addition of an accelerating grid-like structure consisting of a conducting foil electrically insulated from the substrate, provided with holes having their centers coinciding with the tips of the Spindt emitters.
The drawings in FIG. 1 show several electron emitting structures: the perspective view in FIG. 1-a and the corresponding cross-section view in FIG. 1-b show an electron emitter structure consisting of pyramidal Spindt emitters 12 where the holes 15 of the grid foil 14 are square shaped. Said pyramidal emitters are placed over a conducting substrate 11, insulated from the grid sheet by an insulating layer 13. The drawings in FIGS. 1-c and 1-d show cone-shaped Spindt emitters 12′, the holes 15′ in the grid 14′ being circular in this case. In both embodiments shown in FIG. 1, the Spindt emitter tips are substantially on the same plane as the upper face of the grid foil 14 or 14′. The drawings also show that in the embodiments of FIGS. 1-a and 1-b, the conducting substrate is self-supported, while in the one shown in FIGS. 1-c and 1-d, the conducting substrate rests upon an insulating base 10.
As described in the aforementioned documents, electrons are emitted when a negative voltage is applied to the substrate 11, the grid foil voltage being positive. The amount of emitted electrons can be controlled by varying the voltage applied to the grid 14 or 14′. The addition of a separating insulator plate 17 and an anode 16, as shown in FIG. 1-e, yields a triode-like structure. A positive voltage, higher than the grid voltage, is applied to this anode. The assembled parts form a gas-tight chamber 18 which is evacuated.
This basic structure can be used for fashioning lighted panels, in which a transparent anode is coated with a layer of luminescent material—“phosphor”—which emits light when struck by electrons, similarly to what happens in a CRT face.
A problem which occurs with devices of this kind lies in the contamination of the vacuum by gas molecules which are gradually released from the material surfaces. Experimental data show that such devices only operate reliably when the gas pressure inside the evacuated chamber is equal or less than 10−6 torr. With higher pressures, the gas molecules may become ionized; these ions are attracted by and strike the electrically biased surfaces, impairing the emitting structures. Moreover, even when this ionization is absent, gas molecules are adsorbed by the exposed surfaces, modifying the work function of the emitter material and degrading the phosphor layer.
The removal of the molecules from the region in which the electrons travel is achieved by placing inside the evacuated chamber a getter which binds the contaminant gas molecules.
FIG. 2 shows a light emitting display built according to the known technique. Said display comprises a cathode structure composed of a conducting backplate 21 that can be self-supporting or bonded to a rigid insulating slab 20, this backplate being provided with a plurality of Spindt emitters 22 centered at the bottom of through-holes 23 provided in an insulating panel 24 attached to the internal surface of said conducting backplate, the outside surface of said insulating panel being overlaid with a control grid 25 consisting of a conducting foil provided with holes 26 concentric with said through-holes 23 and said emitters 22, the assemblage of the above mentioned elements forming the electron emitting structure or the cathode structure. The display also comprises a rigid transparent front plate 27, usually made of glass, having its internal surface coated with a transparent conducting film 28 (anode); the inside surface of this anode is overlaid with phosphor 29, either as a continuous layer or as a plurality of discrete spots which constitute the picture elements—pixels.
The display shown in FIG. 2 differs from the assembly of FIG. 1-e by the fact that the vacuum chamber comprises the full extension 32 of the device, to allow the displacement, by gaseous diffusion, of the contaminant molecules, from any place in the vacuum chamber to the getter 33 which is placed on a trough 34 provided along one side of the display. This displacement of the gas molecules along the length of the display is called “longitudinal pumping”. The spacing elements between the front plate and the cathode structure in the display of FIG. 2 have been omitted in this drawing for clearness sake.
The pixel definition, specially in the case of colored displays, hinges on the production of sharply defined electron beams, because the defocussing of the beam will result in that a part of the electrons will impinge on phosphor spots of different colors than intended. One of the main causes of this defocussing is the distance travelled by the electrons between the tip of the Spindt emitter and the picture element, i.e., the phosphor spot. In displays built according to known techniques, this distance 35 is about one millimeter, resulting in an unacceptable image quality unless complex and expensive additional structures—not shown in the figure —are used to control the scattering of the electron beams. A more straightforward way of lessening said scattering would be to reduce the distance between the emitter structure and the front plate.
However, this reduction will give rise to a pressure gradient along the display's length, impairing the vacuum in the regions of the display farther from the getter. This effect depends on the relation between the display size 32, typically of the order of 10, 20 or more centimeters, and the free gap 35 between the cathode structure and the front plate. An adequate longitudinal pumping will result only when said gap is equal or greater than 1 millimeter. However, as mentioned before, such large distances require the addition of complex and expensive structures, such as the one described in U.S. Pat. No. 6,013,974.
The approximation between the front plate and the cathode structure constitutes a more straightforward solution for the defocussing problem, due to the fact that the reduction of the path traversed by the electrons before impinging in the front plate will reduce the spot illuminated by the electron beam, which will impinge upon one picture element only, doing away with the need for additional focussing means. However, this nearness diminishes the vaccum conductance, hindering the displacement of contaminant gas molecules along the display length, resulting in a residual pressure gradient. This lack of uniformity in the vacuum quality will bring about the deterioration of the emitter elements as well as of the phosphor, which will be more intense on the central part of the display, resulting in a lack of picture uniformity.